This investigator will study the production of antibodies that inhibit Factor VIII procoagulant function, particularly with regard to regulation by Factor VIII-specific T cells. These inhibitors provide a substantial cause of morbidity for the hemophiliac, and are responsible for the requirement of these individuals for large amounts of blood products including Factor IX which still is responsible for transmission of viral infections. In addition, coagulation concentrates are responsible and may be related to substantial changes in the immune system seen in HIV positive and negative hemophiliacs. This group will study Factor VIII dependent T cell proliferation, cytokine production and expression of activation markers.